High power microwave isolator



W. A. HUGHES HIGH POWER MICROWAVE ISOLATOR Filed Feb. 14, 1955 Jaw/m. Will/d4 A l/ ii, JV

1 Irma 14 Nov. 6, 1962 War/777% 3,063,027 Patented Nov. 6, 1962 3,063,027 HEGHPOWER MICRUWAVE ISOLATOR Willard Aiien Hughes, Los Angeles, Caiiii, assignor to Hughes Aircraft Company, Culver City, Caiifi, a corporation of Delaware Filed Feb. 14, 1955, Ser. No. 487,996 2 Claims. (Cl. 333-242) This invention relates to waveguides for unidirectional microwave transmission, and more particularly to a broadband high power waveguide isolator.

Many utilizations of microwave energy require a power source which is very stable in amplitude and frequency output. For example, the usual microwave generators are susceptible to deterioration with respect to frequency and amplitude output if the nature of the load varies. It follows that their stability is deleteriously affected if energy is reflected from a load which varies in impedance. Accordingly, it is highly desirable to isolate the microwave energy generator from the load. It is also desirable to provide such isolation with a low insertion loss while at the same time exhibiting high loss to the reflected wave from the load.

In the prior art this was most closely achieved by mounting one or more ferrite specimens in a waveguide and subjecting them' to an externally applied static magnetic field. In one form of load isolator a ferrite specimen, preferably of rectangular cross section, is asymmetrically mounted in a rectangular waveguide parallel to its longitudinal axis. The static traverse magnetic field is provided by a relatively large permanent magnet located outside of the waveguide. The magnetic field is channeled through the ferrite, causing the ferrite to provide high attenuation for wave energy passing through it in one direction and low attenuation for wave energy traversing it in the opposite direction.

It is an object of the present invention to provide a microwave isolator of the type described above which is relatively compact and at the same time is capable of handling a large magnitude of power.

It is another object of the present invention to provide a high power microwave isolator that is relatively broadband in its operation and not frequency sensitive.

The desired isolation is achieved in accordance with this invention in the following manner. In a section of waveguide a slender ferrite strip is placed longitudinally in a plane of circular polarized H field. This plane in the waveguide is one parallel to the electric field and its location may be calculated by methods well known in the art. It is the plane in which the transverse component of the H field is equal to the longitudinal component of the H field of the traveling microwave. An external transverse static magnetic field is caused to be focused through the ferrite. With this configuration a forward going microwave signal has a circularly, with a positive sense, polarized H field and the ferrite exerts no absorption effect upon it. Thus, the insertion loss is extremely low. The backward going microwave signal in the same plane has a circularly, With a negative sense, polarized H field and is absorbed by the precessional resonance of the electrons in the ferrite. The undesired reflected microwave energy is then converted to heat by precessional damping losses and the heat is absorbed into the wall of the waveguide. The physical theory concerning the phenomenon of positively and negatively circularly polarized magnetic fields is a rectangular TE mode in a certain plane in the waveguide and the dependence of I the sense of the circular polarization upon direction of propagation is discussed in The Microwave Gyrator,

Bell System Technical Journal, January 1952, pages 1.3l,

by C. L. Hogan.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which a number of embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention. In the drawing:

FIG. 1 is a perspective cut-away view of a simplified embodiment of the invention to aid in the explanation of the invention;

FIG. 2 is a cross-sectional view through a practical embodiment of the invention;

FIG. 3 is a cross-sectional view embodiment of the invention;

FIG. 4a is a longitudinal sectional along the centerline of a third embodiment of the invention;

FIG. 4b is a cross-section taken through the embodiment of FIG. 4a at the plane designated as 4b;

FIG. 5 is a cross-section of a fourth practical embodiment of the invention; and

FIG. 6 is a cross-section of a fifth practical embodimerit of the invention.

Referring now to the drawing, in which like numbers represent like elements in the different figures, it will be assumed for purposes of explanation that it is desired to pass energy from left to right through the isolator. In the cause of simplicity certain structural details have been omitted from the figures. For example, secured to each end of the isolator but omitted from the drawings is a suitable waveguide flange for coupling the device of this invention to other equipment.

Referring specifically to FIG. 1, microwave energy is impressed upon opening 10 of waveguide 12 from a microwave source in a manner well known in the art. Waveguide 12 may be a length of conventional waveguide of the character adapted to be used at the desired microwave frequency. Ferrite strips 14 and 16 are in a conventional manner rigidly secured to the inner horizontal surfaces of waveguide section 12 and are interposed along plane 15, which is the plane of circular polarization of the H field of the traveling microwave. Permanent magnet 18 is constructed and placed so that north pole face 20 and sent pole face 22 focus transverse to the waveguide a narrow magnetic field along the plane 15 through ferrite strips 14 and 16 which are, as shown, of substan tially the same length as the pole faces and of crosssection very small compared With the waveguide crosssection.

Referring now to FIG. 2, permanent magnet 18, shown in cross-section, is placed around waveguide'lz in such a manner that the transverse static magnetic field is focused along plane 15 representing the plane of circular polarization of the H field. Ferrite strips 14 and 16 are secured to the inside walls of waveguide 12 as shown so as to be interposed in the transverse magnetic field along plane 15.

Referring now to FIG. 3, permanent magnet i8 is again shown to substantially surround waveguide section 12 and pole faces 25 and 22 again focus the transverse magnetic field along plane 15 which coincides with the plane of circular polarization of the propagating magnetic field. Longitudinally dividing waveguide section 12 into three substantially equal volumes having substantially equal cross-sections are secured brass cooling fins 24 and 26. As in the previous embodiment, ferrite strips 14 and 16 are mounted along the inside Wall of waveguide 12 in the plane 15 of circular polarization. Mounted in the same plane and on the opposite surfaces of brass of a second practical '3 cooling fin 24 are ferrite strips 27 and 28; and likewise on the opposite surfaces of brass cooling fin 26 and coincident with plane are mounted ferrite strips 29 and 30. Ferrite strips 2730 are of substantially the same dimensions as strips 14 and 16.

Referring to FIGS. 4a and 4b there is shown in section an embodiment of the invention which utilizes a microwave impedance transformer having a broadband impedance matching characteristic. The transformer comprises steps 32, 34, 35, 35', 34, 32 (left to right) in the horizontal walls of waveguide section 31. Ferrite strips 14' and 16 are secured along the length of the narrowest portion of waveguide 31 in proportionally the same position of the cross-section as in the other embodiments of the invention. The ferrite strips may here be smaller in cross-section than are strips 14 and 16 in the previous embodiment.

Permanent magnet 18 with attached pole faces 29 and 22 is placed in a manner to provide the narrow transverse magnetic field along plane 15 through ferrite strips 14 and 16' across the height of the waveguide section 31. Glass or other dielectric seals 36 and 36' are placed across waveguide section 31 at opposite ends of the isolator in a manner so as to sustain a pressure differential between the space inside the isolator and the outside waveguide.

Referring now to FIG. 5, magnet 18 is again shown to substantially surround waveguide 12, and pole faces 20 and 22 focus a transverse magnetic field through the waveguide coincident with plane 15. Ferrite strips 14 and 16 are placed as before along the inside wall of waveguide 12 coincident with plane 15. The strips are here made with a substantially half round cross-section. In addition, cooling fins 3S and 40 are secured on the outside of waveguide section 12 along the length of the isolator coincident with plane 15 and are adapted to conduct and radiate heat away from the ferrite strips 14, 16 and waveguide 12.

Referring now to FIG. 6, waveguide section 12 with ferrite strips 14 and 16 and pole pieces 20 and 22 are shown in substantially the same configuration as in FIG. 2. However, the magnet 18 is reversed to encircle the opposite side of waveguide 12; and external cooling fins 43 in the form of a length of channel 44 are secured along the length of waveguide section 12.

Referring to FIGS. 3 and 5, it is seen that slots 41 and 42 are cut substantially through the waveguide to allow the pole faces to be in closer proximity to the ferrites for purposes of improving the focusing of the transverse magnetic field through the ferrite strips and along plane 15 within the waveguide. The depth of the slot may be the entire thickness of the waveguide wall or any fraction thereof. The horizontal dimensions are substantially those of the ferrite strips. As shown in FIG. 3, pole faces 20 and 22 are accordingly shaped with tongue pro ections of the same dimensions which are inserted into their respective slots. The embodiment according to FIG. 5 utilizes iron cooling fins 38 and 40 for pole faces; and there are adapted to fit directly into the slots.

The permanent magnets adapted for use in the invention may be, for compactness and efficiency, of the well known Alnico type or any similar material having like characteristics. The ferrite strips utilized are fabricated of material suitable for precessional resonance at micro- Wave frequencies. For example, ferrites of the general formula XFe 203 in which X is a bivalent metal ion are particularly useful for this purpose. A suitable ferrite is described in the patent to Luhrs, No. 2,644,930 which consists of 25 moles of manganese oxide, 25 moles of zinc oxide and 50 moles of ferric oxide. The mixture is sintered at 1300 degrees C. The composition and manner of making ferrites having the necessary structure are described in the RCA Review No. 17 for September, 1950 by R. L. Harvey et al. Substances capable of exhibiting the precessional absorption phenomenon at microwave frequencies are known in the art as ferro-magnetic dielectries.

For purposes of description of the operation of the invention it will be assumed that the waveguide is excited in the TB mode. The operation of the invention is not disturbed by this mode. The electric and magnetic fields are set up as usual with the transverse electric field across the narrow dimension of the cross-section of the waveguide and the alternating magnetic fields across the greater dimension of the cross-section of the waveguide and along the length of the waveguide. The electrons in the ferrite strips have an average magnetic moment which is random in the absence of the external magnetic field. When permanent magnet 18 is in place to establish the transverse external field along plane 15, the average magnetic moment tends to line up with the external field which is established. In addition, the alternating magnetic fields, due to the waveguide excitation, cause this average magnetic moment to process. This processing of the average magnetic moment is attended by the production in the ferrite of magnetic fields which tend to reinforce the alternating magnetic fields. The precession frequency depends upon the strength of the external field and when the precessional frequency is made equal to the propagating microwave frequency, precessional resonance occurs.

Under these conditions microwave energy travelling left to right in waveguide section 12 in FIG. 1 is not effected by the precessional resonance and passes unattenuated to the load or other utilization device of the microwave energy. Energy reflected from the utilization device, however, has circularly polarized magnetic fields of opposite sense along plane 15 and in this case precessional resonance occurs to absorb substantially all the reflected energy in the ferrite strips 14- and 16 by precessional damping losses and the energy is absorbed as heat in the wall of the waveguide.

Referring now to FIG. 3, it is seen that the brass cooling fins 24 and 26 of their attached ferrite strips 27 through 30 cause the device to be capable of absorbing and dissipating even higher amounts of energy by virtue of the function of the cooling fins to conduct heat with the outside waveguide walls. The principle of operation, however, remains exactly the same because the cooling fins are at right angles to the electric vector in the waveguide and do not effect the magnetic vector because the fins are made of brass.

The device shown in FIG. 4 is an embodiment of the invention which utilizes an impedance transformer to reduce the dimensions of the waveguide in the region of the ferrite strips so that a smaller magnet may be used due to the reduced magnetic gap between waveguide surfaces through the ferrite strips 14' and 16. The volume enclosed between glass seals 36 and 36 may be either evacuated or pressurized to preclude arcing across the reduced waveguide dimension. This embodiment has the further advantage of providing an even lower insertion loss because less ferrite material is required to achieve a given attenuation.

The embodiment of FIG. 5 utilizes external cooling fins to increase the power dissipating capabilities of the isolator. Because of the proximity of fins 38 and 40 to the ferrite strips 14 and 16, heat is readily conducted from the ferrite strips to the cooling fins. In this embodiment the cooling fins function also as magnetic pole faces and thus are shaped in a conventional manner to focus a strong magnetic field along plane 15 through the ferrite strips. The half-round cross-section of ferrite strips 14 and 16 provides the advantage of decreasing the tendency of the microwave energy to are between the ferrites because of the rounded surfaces. Also the rounding provides a broader band of operation because of internal demagnetization due to the rounded geometry.

Referring to the embodiment according to FIG. 6, cooling fins 43 attached as a channel 44 to the waveguide afford another practical means for dissipating the heat from the ferrite strips. Magnet 18 is reversed from the previous embodiments in order that the external cooling fins of channel 44 may be closer to the ferrite strips.

In a specific structure constructed according to the embodiment of FIG. 5, the following specifications were used:

The brass waveguide section was 3" including flanges; the inside were 1.122" by .497";

in overall length dimensions of the waveguide the ferrite strips Were .100" x .070 x 1.7" and were placed so that their centerlines were .220 from the closest vertical wall; cooling fin-pole pieces 38 and 40 were of soft pure iron .18 x .55" x 1.7 each with a tongue .062" x 1.7" inserted through a longitudinal slot of the same dimensions in the waveguide walls to make actual contact with the ferrite strips along their entire length; the magnet was horseshoe in crosssection and 1.7" long and cast of Alnico V material; the magnetic field at the pole faces was 3700 Gauss and at the surface of the ferrites opposite the pole faces, 3300 Gauss.

Using a pulsed carrier over the band from 8500 ki1omegacycles to 9500 kilomegacycles, the pulses being 2.4 micro-seconds in width and of 4-15 cycles repetition rate continuous duty, the attenuation in the forward direction was .5 db while the attenuation in the reverse direction was 12 dbil db over the above band. Thus an attenuations ratio of nearly 30 to l in db was achieved while transferring power peaks of greater than 300 kilowatts and average power of greater than 300 watts.

Other advantages over the prior art are that a lower voltage standing wave ratio (VSWR) is seen by the microwave circuitry on the input side of the isolator; the isolator of this invention operates over a considerably broader band and is, therefore, much less frequency sensitive; the insertion loss of the device is negligible (.5 db); and the power handling capabilities are at least 100 times better than in the prior art systems. It is also seen that the device is simple in construction so as to ofier economic advantages.

What is claimed is:

1. A waveguide isolator device utilizing the principle of magnetic precessional resonance and the inherent energy dissipation associated therewith in a ferrite substance for unidirectionally absorbing microwave energy propagating within the waveguide, said isolator device comprising: a section of rectangular waveguide being adapted to be excited in a conventional plane polarized mode; a number or" cooling fins, said cooling fins being thin heat conductive nonpermeable sheets supported along the length of said waveguide internal thereto and disposed parallel to the magnetic field in a manner to avoid disturbance of the electric field; a first plurality of ferrite strips supported along and in contact with said cooling fins and made coincident with a plane of circular polarization of the magnetic field within the waveguide; a second plurality of ferrite strips secured along the interior walls of said waveguide section also coincident with the plane of circular polarization in the magnetic field; an external magnet adapted to provide a transverse magnetic field focused along and throughout said plane of circular polarization.

2. A microwave isolator device in which the effect of microwave energy loss through heat dissipation due to electron precessional resonance damping losses in a ferrite substance is utilized to allow propagation of microwave energy in a waveguide in one direction and preclude propagation of the microwave energy in the opposite direction, said device comprising: a length of rectangular waveguide being adapted to be excited in the plane polarized mode and having thereby a plane throughout the length of the waveguide section along which the magnetic vector is circularly polarized; a pair of slender ferrite strips secured along the length of said waveguide section in contact with the inner surface of opposite walls thereof along said plane and having a semi-circular cross-section, the curved portions of said strips facing each other and the ilat portions of said strips being disposed perpendicular to said plane; a permanent magnet external to the waveguide for producing a transverse magnetic field along the plane of circularly polarized magnetic field through said ferrite strips; cooling fins external to said waveguide section and attached thereto consisting of a material suitable for serving also as pole faces for said magnet and being attached thereto, each of said cooling fins being secured to said waveguide coincident with the plane of circular polarization external to said waveguide section and being secured thereto in a manner such that each of said cooling fins is afiixed to one of said ferrite strips for conducting heat therefrom into space.

References \Cited in the file of this patent UNITED STATES PATENTS 2,643,296 Hansen June 23, 1953 2,648,047 Hollingsworth Aug. 4, 1953 2,748,353 Hogan May 29, 1956 2,745,069 Hewitt May 8, 1956 2,767,380 Zobel Oct. 16, 1956 2,776,412 Sparling Jan. 1, 1957 2,777,906 Shockley Jan. 15, 1957 2,844,789 Allen July 22, 1958 2,849,684 Miller Aug. 26, 1958 OTHER REFERENCES Journal of Applied Physics, vol. 24, No. 6, June 1953, pages 816-817.

Fox et al.: Behavior and Applications of Ferrites, Bell Technical Journal, vol. XXXIV, No. 1, January 1955. 

